The present invention relates to a process for the formation and moisture extraction or drying of porous filter cake comprising individual solid particles on a filter medium by means of a fluid acting on the filter cake and having a differential pressure compared with the filtrate located behind the filter medium.
Cake-forming filtration is used e.g. in the preparation or conditioning of raw materials, such as ores, coal, etc., during the separation of sludges and slurries and similar processes. Conventionally the suspension is separated by immersing a filter medium, e.g. a monofilament, multifilament or a filter cloth and depositing the solid as a filter cake on the filter medium. Up to now the pores of the cloth have had a much larger diameter than most of the solid particles of the suspension, so that at the start of deposition a solid penetration through the filter medium occurs, which is undesired and must therefore be kept as small as possible. After solid particles have been attached to the filter medium for a certain time, the forming filter cake has narrower pores or capillaries than the filter medium and therefore holds back the further fine solid particles and itself acts as a filter.
After emerging from the suspension, this filter cake is dried. During vacuum filtration a fluid, i.e. another liquid or a gas and preferably air is sucked through the bed, or is forced through it in the case of pressure filtration. Moisture is initially extracted from the coarsest capillaries of the filter cake having a low intake capillary pressure and they are followed by the smaller capillaries with a higher intake capillary pressure. The drying process is continued until a requisite residual moisture content in the cake is reached. During the drying of the narrower capillaries, the drying fluid already flows through the emptied, larger capillaries and through the filter medium, so that it is only possible to maintain the pressure potential with increased energy expenditure on the part of the pressure unit. The drying fluid volume flow flowing through the cake during the drying time is a measure of the economics of the filtration process. In particular ultra-fine grain-rich or compressible filter cakes have a craking tendency during moisture extraction. The cracks extending from the cake surface to the filter medium cause a considerable drying fluid throughput, so that obtaining the pressure difference leads to an extremely high work expenditure and a high fluid throughput, which is extremely uneconomic.
Attempts have been made to reduce cracking in that the filter medium is subdivided into a plurality of small filter surfaces, between each of which is arranged a relatively wide separating web, so that the tensions occurring in the bed due to the capillary forces are reliably transferred up to the particular separating web and can be broken down or reduced there through the shrinkage of the cake. However, this measure suffers from the disadvantage that the separating webs lead to a considerable filter surface loss. Attempts have also been made to dry the filter cake by means of diaphragms or membranes, the latter being mainly constituted by sintered glass or ceramic filter plates. However, these filter plates suffer from the disadvantage that, as a result of their thickness and characteristics, they have such a high flow resistance, that the industrial use thereof is impossible. Due to the necessary high pressure difference, it is only possible to use pressure filtration, because with vacuum filtration it is only possible to reach pressures of 0.8 to 0.9 bar due to the vapour pressure of the liquid.
A further disadvantageous effect occurs with a suspension having dissolved salts. Following the expulsion of the suspension from the filter cake, the drying fluid, which is generally a gas flows through the filter cloth. However, this gas dries the filter cloth and cools it due to the removed evaporation enthalpy of the suspension. A further cooling occurs as a result of the gaseous state law due to the expansion of a gaseous fluid on flowing through the bed. Crystallization out of the dissolved salts occurs, so that the cloth gradually becomes clogged with the crystallized out salts and no longer has any filtering effect after a short time.